The sound that we perceive in a space is a combination of the direct sound, early reflections from boundary surfaces and elements in the room and the later arriving dense reflections forming the reverberant sound. Consequently, the control of reflections is essential to providing an acoustical environment suitable for the purpose of the space. Reflections can be controlled by only three types of surface treatment, namely absorption, reflection and diffusion. Sound is attenuated by absorption, redirected by reflection and uniformly scattered by diffusion.
Absorption is the most common type and, in fact, most people refer to acoustical surfaces as absorptive surfaces. This is unfortunate because reflective and diffusive surfaces are equally important. Examples of high-mid frequency absorbing surfaces include acoustical ceiling tile, fabric wrapped fiberglass panels, drapery, carpeting, etc.
Low frequency absorbers include membrane and Helmholtz resonators. Reflecting surfaces include any flat surface, such as drywall, glass, etc, in which the reflected sound is directed at an angle equal to the angle of incidence. Diffusing surfaces have historically included shapes used in classic architecture, such as statuary, coffered ceilings, relief ornamentation, columns, etc. While these surfaces are useful and beautiful, they scatter sound over a limited range of frequencies.
Beginning in the early 1980s, new types of mathematically designed diffusing surfaces were introduced. These included reflection phase gratings, binary and ternary amplitude gratings and optimized surface shapes, all of which are reviewed below.
Traditional mineral fiber or fiberglass ceiling tile and fabric wrapped panels offer reflection and reverberation control by absorbing sound. Typically, these porous materials preferentially absorb sound in the high and mid frequencies, because the absorption mechanism is based on particle velocity and air movement is low near or on a boundary surface. Absorption is improved if there is an air cavity between the boundary and the porous material. While these conventional absorptive surfaces are useful in controlling high and mid frequency reflections, they can introduce an acoustically “dead” space, because they are removing the ambient sound frequencies. In addition, because they offer minimal low frequency absorption, preferential removal of high frequencies can create the impression of a reverberant space that contains excessive low frequencies and hence can sound “boomy”. This situation is further exacerbated by the fact that most spaces contain additional surfaces that absorb high frequencies, such as carpeting, drapery and people. Hence, exclusive use of traditional absorptive ceiling tile and fabric wrapped panels can tilt the reverberation response of a room such that the highs are attenuated and the lows are untreated.
What would solve this problem is to develop a tile and panel that offers pure high frequency diffusion without absorption down to a specified cross-over frequency and then transitions into pure absorption below this frequency. Several attempts have been described using binary and ternary sequences to accomplish this. The limitation of these sequences is the fact that they include a binary zero, which introduces unwanted accompanying absorption to the high frequency diffusion, thus reducing its effectiveness. The present invention solves this problem by describing a new class of acoustical ceiling tile and wall panels which provides high frequency diffusion, without any accompanying sound absorption, above a specified cross-over frequency, and then transitions into a pure absorber below the cross-over frequency. In some cases, the diffusive surface is designed using a mathematical number theory sequence devoid of the binary zero (0) to preclude sound absorption above the transition frequency. The absorption cross-over frequency can be designed to extend in a range from mid to low frequencies. This new transitional panel essentially is the “holy grail” of acoustical panels and offers the possibility to control reflections and reverberation in a manner that is desirable for almost any type of space including, classrooms, lecture halls, meeting rooms, offices, rehearsal spaces, performance spaces, recording/broadcast studios and commercial and home theaters. In effect, these novel surfaces overcome the limitations of traditional acoustical ceiling tile and fabric wrapped panels and can result in higher speech intelligibility and an enhanced space to perform and audition music.
Reflection Phase Gratings: Diffusion without Absorption:
Diffusors can be used to improve the acoustics of enclosed spaces to enhance the experience of listening to music and make speech more intelligible. Early research in diffusors began by considering non-absorbing reflection phase grating surfaces, such as Schroeder diffusors. These surfaces consist of a series of wells of the same width and differing depths. The wells are separated by thin dividers. The depths of the wells are determined by a mathematical number theory sequence that has a flat power spectrum, such as a quadratic residue or primitive root sequence. Limitations of these early diffusors, including periodicity effects and flat plate frequencies, have been mitigated by a recent invention, U.S. Pat. No. 6,772,859, using embodiments of aperiodic tiling of a single optimized asymmetric diffusive base shape.
Binary Amplitude Diffusors: Diffusion with Accompanying High Frequency Absorption.
More recent research has concerned the development of hybrid absorber-diffusors; these are surfaces that are referred to as amplitude gratings and contain reflective and absorptive areas. The location of these areas is determined by binary mathematical sequences in which a 1 refers to a reflecting area and a 0 to an absorptive area or vice versa. As opposed to the reflection phase gratings, these binary amplitude absorber-diffusors inherently absorb-roughly 50% of the incident sound and diffuse the remaining energy. Applicant has described effective planar and optimized curvilinear two-dimensional binary amplitude sequences in U.S. Pat. Nos. 5,817,992 and 6,112,852, respectively. A problem with hybrid absorber-diffusors is that energy can only be removed from the specular reflection by absorption. While there is diffraction caused by the impedance discontinuities between the hard and soft patches, this is not a dominant mechanism except at low frequencies. Even with the most optimal arrangement of patches, at high frequencies where the patch becomes smaller than half the wavelength, the specular reflection is only attenuated by roughly 7 dB.
Ternary Diffusors: Improved Diffusion with Accompanying High Frequency Absorption.
If it were possible to exploit interference, by reflecting waves out of phase with the specular lobe, then it would be possible to diminish the specular lobe further. Applicant has found that this can be achieved by using a new class of hybrid diffusors combining the aspects of an amplitude grating with those of a reflection phase grating. These new surfaces contain the elements of an amplitude grating, namely reflective and absorptive patches, with the addition of additional reflective patches, in the form of wells a quarter wavelength deep at a specified design frequency, which can constructively interfere with the zero-depth reflective patches. The simplest form of these hybrid gratings is an absorber-diffusor with a random or pseudo-random distribution. But a more effective design is based on a ternary sequence, which nominally has surface reflection coefficients of 0, 1 and −1. The wells with the pressure reflection coefficient of −1 typically have a depth of a quarter of a wavelength at the design frequency and odd multiples of this frequency to produce waves out of phase with those producing the specular lobe, i.e. the wells with a pressure reflection coefficient of +1. This results in a better reduction of the specular reflection. Ternary sequences are therefore an extension of the binary amplitude diffusor and are an alternative way of forming hybrid absorber-diffusors, which achieve superior scattering performance for a similar amount of absorption, as the BAD panel. Applicant has described these hybrid amplitude-phase grating diffusors in U.S. Pat. No. 7,428,948 B2.
While interference effects in the ternary diffusors improve the diffusion by lowering the specular component, they still provide some unavoidable high frequency absorption, due to the pressure reflection coefficients of 0 in the sequence. This absorption reduces the amount of incident sound that can be constructively diffused or scattered. Therefore, it would be significant to remove all of the high frequency absorption above a certain transition frequency, so all of the incident sound could be diffused. In addition, the absorption of conventional acoustical ceiling tile and fabric wrapped panels is heavily weighted to the mid and high frequencies, which can often leave a space feeling acoustically “dead”, because they typically only remove the frequencies above 1,000 Hz range which generate a sense of ambiance and contribute to speech intelligibility. Since other elements in the room, such as drapery, carpeting and people also absorb preferentially in the high-mid frequency region, additional absorption of acoustical ceiling tile and fabric wrapped panels, often further unbalance the reverberation time and can actually give the perception of too much low frequency sound in the room. While these acoustical ceiling tiles and fabric wrapped panels may prevent complicating reflections, what is really needed is a ceiling tile or panel that diffuses the high frequencies, thus removing interfering reflections while maintaining the sound as ambiance, and absorbs the mid and low frequencies, which are perceived as low frequency reverberance or boominess. This reverberation can reduce speech intelligibility by decreasing the signal (the wanted information) to noise (the reverberation) ratio.
The present invention describes a new technique to solve this problem. The approach is to return to the concept of the original non-absorbing reflection phase gratings, with a significant difference. Instead of making the diffusors non-absorbing over a wide frequency range, the present invention diffuses incident energy above a transition frequency and absorbs the remaining transmitted energy below the transition frequency by a number of approaches as will be explained hereinafter.